Fivefold symmetric homonuclear dipolar recoupling in rotating solids: Application to double quantum spectroscopy

We describe a new fivefold symmetric approach to homonuclear recoupling in rotating solids that is based on rotor-synchronized, spin-lock rf irradiati on of the type employed previously in MELODRAMA and C7 (and their derivativ e sequences) for C-13-C-13 recoupling. The fivefold sequence, like its seve nfold relatives, is gamma-encoded, and therefore exhibits a theoretical eff iciency of 73% for double quantum filtering (2QF). However, since the ratio of rf field strength, omega(rf)/2 pi, to spinning frequency, omega(r)/2 pi , is lower, it is possible to operate the sequence at higher spinning rates , and we have investigated the 2QF efficiency as a function of H-1 decoupli ng field strength at high spinning frequencies. We observe dramatic oscilla tions of the recoupled signal with a period similar to omega(r)/2 pi indica ting that the H-1 reservoir is behaving partially inhomogeneously. This kin d of double quantum recoupling is explored in multiple spin systems and we derive analytical forms for polarization transfer and double quantum excita tion relevant for uniformly labeled systems. Finally, the wide applicabilit y of the fivefold sequence is demonstrated with INADEQUATE type spectra of uniformly C-13 labeled sucrose and L-alanine. (C) 1999 American Institute o f Physics. [S0021-9606(99)01615-3].